The novel's protagonist a brooding, reclusive detective always solved the case in the end.
Correct: A
The phrase 'a brooding, reclusive detective' is an appositive that renames or explains 'the novel's protagonist'. This appositive is non-essential information and should be set off by commas. Option A correctly places a comma before and after the appositive phrase. The adjectives 'brooding' and 'reclusive' are coordinate, so a comma is correctly placed between them.